Missing Number from array with duplicates(Method - 2)

#include <bits/stdc++.h>
using namespace std;

int main()
{
// Missing number from array with duplicates

vector<int>v={1,3,5,3,4};
v.insert(v.begin(),767);
int n=v.size();
int duplicate;
int missing;
for(int i=1;i<n;i++){
int index = abs(v[i]);
if(v[index]>0){
v[index] *= -1;
}
else duplicate=index;
}

for(int i=1;i<n;i++){
if(v[i]>0){
missing = i;
break;
}
}
cout<<"duplicate number is "<<duplicate<<endl;
cout<<"missing number is "<<missing<<endl;

return 0;
}